Something moving in a circle has a uniform speed but not velocity. The acceleration is towards the center.
Acceleration is a change in speed or direction. So if we can't change the speed, we have to change the direction.
For objects undergoing uniform circular motion, the direction of the object is constantly changing, the speed is constant, and the acceleration is towards the center of the circle.
The speed of an object in uniform circular motion is given by:
Where r is the radius of the circle and T is the period (the time it takes to compete one circle).
This video is one of my favorite examples:
Notice that you have to keep hitting the ball (applying a force), and thus causing an acceleration towards the center to keep the ball going in a circle.